In an L-Jetronix type fuel injection apparatus provided with a fuel injector in the vicinity of each cylinder, the intake air volume aspirated in each cylinder is detected by an air flow meter provided in an intake manifold, and a basic injection pulse width Tp of the fuel injector is determined from this intake air volume Qa and the engine speed Ne detected by the crank angle sensor or other means using the following equation: EQU Tp=K.multidot.Qa/Ne (K is a constant)
However, due to the distance between the air flow meter and a cylinder, and the capacity of the intake manifold, the air flowrate aspirated into the cylinder does not immediately follow increases and decreases at the position of the air flow meter but follows them after a certain delay.
In order to make the fuel injection amount correspond accurately to the actual cylinder intake air volume, a fuel injection control apparatus wherein fuel injection is performed based on a weighted average value Avtp is disclosed in, for example, Tokkai Hei 3-111639 published by the Japanese Patent Office. In this publication the weighted average value Avtp is calculated by the following equation: EQU Avtp=Tp.multidot.Fload+old Avtp.multidot.(1-Fload) (1)
where Fload is a weighting average coefficient which specifies the aforesaid delay. Further, the term "old" in the equation represents a value obtained by the immediately preceding calculation.
In the aforesaid apparatus, variations of the cylinder intake air volume are predicted by repeatedly computing equation (1) with a certain time period. Discrepancies between the air flowrate detected by the air flow meter and the cylinder air intake volume are particularly apt to occur during transient engine running states. If fuel injection is performed according to the cylinder air intake volume predicted as described hereinabove, however, the air-fuel ratio (AFR) of the air-fuel mixture which is actually burnt can be accurately controlled to a target value.
Fload is determined by previously setting up a map depending on the engine speed Ne and engine load, and referring to this map each time the calculation is performed. To determine the engine load, data for throttle opening TVO or the like may be used.
To increase the accuracy of correspondence between fuel injection amount and cylinder air intake volume in the aforesaid apparatus, the number of grid points on the map must be increased. The map is however set up by carrying out experiments on each type of engine, and a large operating time is consequently required to set up a map having the requisite precision.
Further, if the time required to compute Tp by detecting Qa using the air flow meter is long, a difference appears between the phase of Avtp found by applying a weighted average to the computed value of Tp and the phase of the actual air volume aspirated in the cylinder. As a result, during acceleration for example, the increase of Avtp is delayed so that the first part of the fuel injection amount tends to be insufficient, and the subsequent overshoot causes the later part of the fuel injection amount to be excessive.